One of the problems of exploration of Mars (for example) is safe landing. The technology, method and concept of landing the rovers is well known - but the problems to me seem to be caused by landing vertical.

Might it be possible to find a working technolgy to land horizontally on Mars by using a kind of ski? They would have to resist to rocks and to be able to handle dunes - for example. So normal ski like those known from sports cannot be used. In act I'm thinking of something similar to the things helicopters a standing on when at ground (again I'm missing the right english term - in german they are called "Kufen").

Might it be possible to develop something useful and new out of this raw idea?

Stimulation was my intention. Existing skids won't work but get destroyed I suppose. But it might be possible to develop something similar that will work. It could be tested in the deserts and in the Antarctis.

I had SS1 in mind when I started thinking of it and supposed that horizonal landing of robots etc. at Mars, moon and the jovian moons might be more safe perhaps.

Then the skids would be the first problem - the next will be to handle the dangers and problems caused by the craters. Much of them have to be avoided somehow. The technologies to do that in principle are existing already or are under development - the hazcam and -software of the Mars rovers for example, some technologies tested by DS1 may be another example.

Orbiters could analyze the surface by looking at and scanning the surface not vertical below them but looking tangentially...

The problem of horizontal landing on Mars is quite well outlined in the Mars section of X-Plane; the biggest problem is the lof atmospheric pressure. The Indicated speed is about 1/10th of true airspeed; thus, if you have a plane that could land at 30 knots in earth atmosphere, it would be doing 300 on Mars! The braking would be a problem too, with gravity of just 0,4G.

NASA proposed a martian glider for 2003 (too bad they missed the launch window). It would have landed the same way as free-flight F1A1 gliders; by dethermalizing the tailplane (essentially what Space Ship One did) and gently stalling the plane on the ground. It would not likely work with heavier planes though.

The problem of horizontal landing on Mars is quite well outlined in the Mars section of X-Plane; the biggest problem is the lof atmospheric pressure. The Indicated speed is about 1/10th of true airspeed; thus, if you have a plane that could land at 30 knots in earth atmosphere, it would be doing 300 on Mars! The braking would be a problem too, with gravity of just 0,4G.

Your IAS story is plain nonsence of course. Sure, the pressure is lower so you need a higher speed to get the same lift, but that has little to do with Indicated vs. true airspeed. If you plan to land on the mars surface, you would not use a speedometer build for terran pressures, would you?
What's more, the lower gravity would reduce the amount of lift you need, and thus the speed you need to fly. The lift can be expressed as:

L = C * 1/2 rho V²

with L the lift, C a factor containing the wingarea, the angle of attack and the properties of the wings profile, rho the density of tha atmosphere and V the speed.

So, how much bigger (with equal C) does V need to be on mars? If we fill in your figures and keep the rest a constant, we can simpify to the equation below. Let's solve x in the equation:
0,4 = 0,1 (x)² <=>
(x)² = 0,4 / 0,1 = 4 <=>
x = 2.

So, if the figures you say (0,4 G and 1/10th atmospheric density) are correct, the speed to fly on mars will be twice the speed you need on earth. So, to keep it managable in a landing, you'll need a rather big wing area to get to a low weight to wing area ratio, and thus a low landing speed (and propably a relatively high drag, wich is an advantage in this scenario). A rule of thumb is that something can fly slower if the mass/wing area ratio is smaller.

I do agree that braking will be harder once landed. Not only will friction be lower due to a lower G, also aerodynamic breaking will be harder due to the lower atmospheric density.

I think a real flying landing would be pretty hard to realize. Flying is difficult to do automaticly.

First of all, those are NOT MY FIGURES, but YOURS. I did not state the air density was 1/10th of that on earth. I said that at 300 knots true speed (ground speed in still atmosphere) the indicated air speed was one tenth. The actual atmospheric density is about HALF A PERCENT of what it is on earth. Check your calculations on that.

Second, TAS tells you the actual speed of ambient atmoshperic mass. It DOES NOT MATTER if the atmoshpere is on earth or mars. If TAS tells you you're doing 300 knots, you are doing 300 knots. Period.

I think one of the few ways you could ensure safe horizontal landing on another planet without building a runway would be to use directional thrusters to slow the craft down to almost a stand still, for example, the Harriers' semi-horizontal landing... I've seen a few land so slowly because of their thrusters that cars were passing by them with no problem...

Even if that wouldnt work on a space craft, thrusters are such an amazing idea arent they?

Thrust vectoring or the lift fans on the F-35B would work (or perhaps a rocket version of the V-22 design). The Harrier design wouldn't work due to the fact that it overheats the engine tremendously, and the water that they carry for that only lasts for 90 seconds and severely reduces their payload.

First of all, those are NOT MY FIGURES, but YOURS. I did not state the air density was 1/10th of that on earth. I said that at 300 knots true speed (ground speed in still atmosphere) the indicated air speed was one tenth. The actual atmospheric density is about HALF A PERCENT of what it is on earth. Check your calculations on that.

Second, TAS tells you the actual speed of ambient atmoshperic mass. It DOES NOT MATTER if the atmoshpere is on earth or mars. If TAS tells you you're doing 300 knots, you are doing 300 knots. Period.

Yes, I mixed up the 10%. No need to start yelling. However, you did talk about Indicated airspeed, not true airspeed. Of course if TAS sais 300 knots, you're doing 300 knots. Duh, that's by definition...

The speed at 0,5% density would be sqrt(0,4/0,005) = 9 times the speed on earth. That *is* going to be a problem at landing.

Your claim that you'd get an indicated airspeed (I suspect using a speedometer designed for earth pressures?) of 30 knots doesn't sound right. The pressure caused by the speed is 1/2 rho V², why would it only be 10 times lower if you claim that the density is only 0,5%? And, why would you go to mars using a speedometer that's constructed for earth? I'd take one that gives an IAS that's reasonably close to TAS at ground level, just like I do on earth.

If you are right that the density is that low, the problem is not that IAS is so much off, but that flying itself becomes a problem with reasonably sized wings, especially since the density is bound to drop even lower at higher altitudes.

Then, you call an airspeed indicator a speedometer. You will find a speedometer in a family sedan, not an airplane.

Indicated airspeed tells what kind of dynamic pressure the wing sees. If the IAS is 30 knots, the wing will perform accordingly, regardless whether on earth or on mars or in jupiter. The gravity will affect the plane differently, but that's beside the point. IAS has everything to do with how the plane flies. (Angle of attack will ultimately tell if the plane will fly or not, but the IAS tells a lot of how the plane will behave at the given load.)

Then, you call an airspeed indicator a speedometer. You will find a speedometer in a family sedan, not an airplane.

A speedometer is a device measuring speed. How it works is irrelevant. However, since English is not my native language, I may have used a word that causes confusion. I am vey well aware how instruments in airplanes work. I own one (an airplane, not just an airspeed indicator, that is).

Quote:

Indicated airspeed tells what kind of dynamic pressure the wing sees. If the IAS is 30 knots, the wing will perform accordingly, regardless whether on earth or on mars or in jupiter. The gravity will affect the plane differently, but that's beside the point. IAS has everything to do with how the plane flies. (Angle of attack will ultimately tell if the plane will fly or not, but the IAS tells a lot of how the plane will behave at the given load.)

No, it will not actually. If the atmosphere is a thin as you say (I did not check), it will probably both stall and exceed over Vne at the same time. Your story about how IAS is lower than TAS but you can use it all the same because the pressure influences the characteristics of the airplane in the same way is only valid at relatively low altitudes/high pressures. While (indicated) stall speed stays constant, (indicated) Vne drops with altitude!
The fact that there is lower gravity is not besides the point, as gravity has everything to do with being able to fly or not, with wingloading, required lift, etc.

Last edited by André on Thu Oct 21, 2004 7:34 am, edited 1 time in total.

What about braking rockets, directional thrusters as Cathleen too is asking?.

I have been thinking horizontally not only at Mars but on the moon and the jovian moons too. There are no atmospheres - so directional thrusters and braking rockets are required.

I think a horizontal landing in that case is only complicating the whole procedure. Why would you want to deal with a horizontal speed if you could also just focus on the vertical speed? Airplanes only land horizontally because they need horizontal speed to stay in the air and stay controllable (ok, the Harrier and JSF are exceptions, but they are very complicated). However, all measures are taken to keep this speed as low as possible (for instance, by using flaps). Landing with a lower (ground) speed is easier and saver, that's the reason airplanes prefer to start and land against the wind.

So, if you're going to use rockets or thrusters to stay up anyway, I would keep matters as simple as possible and land vertically.